2. Organ: Group of different Tissues working together.
Leaf: 3 different Functions: Photosynthesis, Gas Exchange and Transpiration.

3. Tissue - Group of cells of the same kind = they have the same shape and function. eg: Phloem - Cell that transports "sap"
eg: Xylem - Transports water and minerals.
eg: Plaside - Carries out P/S

4. Cells: multicelluler organinisum.
a) In development of the organism, the cell specializes: they switch on genes which change their shape giving the cell a particular function: This is called Cells differentiation